Objective: To understand the heterogeneity among lymphocyte populations with regard to function and differentiation. Also to study the properties of the molecules which mediate the interaction between T-lymphocytes and B-lymphocytes. Approach: Cell populations from normal or immunized animals are prepared from lymphoid tissues using a variety of techniques: 1) treatment with antisera against surface antigens (0, Ig, Ia, etc.) plus complement in vitro. 2) Differential sensitivity to anti-thymocyte serum in vivo. 3) Differential half-life in vivo. 4) Differential sedimentation. 5) Removal of adherent cells on columns of G-10 Sephadex. These partially purified populations are tested in vitro for their ability to interact with antigen and perform a variety of functions such as antibody production, helper activity, lymphokine production, and cytotoxicity. Cell interactions between subpopulations are studied either directly or with cell free products. We have developed a number of microtechniques which allow us to work with small numbers of cells in culture and in fact in some cases detect the activity of single cells. In such cases we are able to examine two activities at limiting dilution to determine if they are the products of the same or different cells.